1. Field of the Invention
The present invention relates to an image forming method used in digital printers, digital copiers, and the like in which an image is treated as digital signals, and also to an image forming device and an electrostatic latent image developer. In particular, the present invention relates to an image forming method, an image forming device and an electrostatic latent image developer using a two-component developer in which a toner is mixed with a magnetic carrier.
2. Description of the Related Art
Methods for making image information visible through an electrostatic latent image, such as an electorophotograohic method, are currently used in various fields. The electrophotographic method comprises forming an electrostatic latent image on a photorecepter in charging and exposure steps, and developing the electrostatic latent image by using a developer containing a toner such that the image becomes visible in transferring and fixing steps.
In a digital image forming device, binary information of two values for "ON" and "OFF" is given as two-dimensional information for a predetermined point on a photorecepter on the basis of character and image data. When such a system is used to record a halftone image, an area modulation method using a dot structure or an innumerable line structure utilizing parallel ten-thousand or more lines has been utilized in many printers and copiers using a digital electrophotographic system because of the relatively simple algorithm and low costs.
In an image forming device, using an electrophotographic system to reproduce multiple gradations, and in such a color image forming device in particular, the developer may be a two-component developer consisting of a toner and a carrier, or a single-component developer such as a magnetic toner which is used alone. The two-component developer is widely used at present because the carrier bears its share of functions, such as agitation, transferring, and charging of the developer. Thus, the two-component developer with its share of functions can provide stable chargeability and high controllability.
As for a developing method, a cascade method was used formerly. At present, the dominant developing method is a magnetic brushing method using a magnetic roll as a carrying member for the developer. Conductive magnetic brushing (CMB) development using a conductive carrier and insulating magnetic brushing (IMB) development using an insulating carrier are known as examples of two-component magnetic brushing development.
IMB development has the features that the relation between the latent image potential and the image density on a photorecepter is linear, and the slope of the linear line indicating the relation is small, but on the other hand, the padding of the solid made from tonner is not supplied well and the edge effect is great. CMB development has the features that there is no edge effect and the padding of the solid made from tonner is supplied well in contrast to the insulating magnetic brushing development. However, the relationship between the latent image potential and the image density on a photorecepter has a steep slope, and there is the drawback that bias leaks cause breaking of the latent image resulting in it being easy for brush marks to be caused. In particular, image defects due to brush marks significantly impair the quality of color images.
When a black-and-white image is formed by using only a black toner, these problems have no significant effect on the functional image quality if the level of these problems is low. However, these problems prove to be serious when colored toners are overlapped to form a color image. This is because the effect of the above problems in a black-and white image is confined to microscopic changes in density, but in a color image, is expressed as microscopic changes in the color tone, such that noises with different colors are created in a gradation image. The above problems therefore have a significantly adverse effect especially on the functional image quality of a color image.
Several conductive magnetic brushing methods have been disclosed which can solve the above problems and in which the padding of the solid made from tonner is supplied well, the edge effect is reduced and it is difficult to produce brush mark.
For example, Japanese Patent Application Publication (JP-B) No. 7-120,086 discloses a carrier which is produced by coating a core material (hereinafter called "carrier core" or in some cases simply "core") having a relatively low resistance with a highly resistant resin, and is thereby characterized in that the electric resistance thereof rapidly changes in a certain electric field, and in that higher resistances are exhibited in lower electric fields and lower resistances are exhibited in higher electric fields. JP-B No. 7-120,086 explains that since a high electric field is applied to a latent image portion and a low electric field is applied to the portions other than the latent image portion, solid black areas can be printed well, and at the same time, there is no excess application of carrier to the non-latent-image portions. However, judging from the description in the "Examples" and the "Operation" of JP-B No. 7-126,086, it can be assumed that in the invention of JP-B No. 7-126,086, the core having low resistance is partially exposed because the thickness of the coated resin layer is considerably thin. It is thought that such a structure causes the resistance to decrease in high electric fields. In practice, as shown in the comparative examples which will be described later, the electric resistance of a carrier whose a core material was completely coated with a thick resin coating layer was high even in high electric fields, and a good solid image could not be obtained. In the above partially coated carrier in which a part of the low resistance core is exposed, charges easily moves via the exposed surface which makes it easy for brush marks to be formed on the latent image portions.
Ferrites which have relatively low electric resistance and micro-irregularities formed by primary particles on the surface thereof are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 61-107257 and 61-130959. These disclosures explain that the provision of such a micro-irregularities suppresses leakage between differently polarized charges and thus prevents the occurrence of brush marks. However, because micro-irregularities are formed on the surface of the carrier, the contact area with the toner increases, giving rise to the problem that the toner is easily adsorbed on the surface of the carrier, and as a result, the charge-donating capability of the carrier deteriorates over time.
In addition, JP-A No. 6-161,157 discloses a material in which the ratio of the electric resistance of a core material of a resin-coated carrier to the electric resistance of the carrier itself is prescribed. It is shown therein that this material satisfies all of resolution, solid image density and reproducibility of fine lines simultaneously. However, a sufficient effect on the prevention of occurrence of image defects especially in color images was not exhibited.
As described above , as for CMB development, judging on the basis of recent stringent requirements for high quality of color images and the like, none of the conventional image forming methods are sufficient with respect to image defects relating to conductive magnetic brushing, specifically, to brush marks due to the breaking of a latent image caused by bias leakage.